Conventionally, on a seat of a vehicle, an occupant-restraining seat belt apparatus is provided that inhibits the occupant from being thrown forward due to the inertial force when a vehicle stops suddenly or an impact such as a collision occurs. Usually, such a seat belt apparatus has an emergency-automatic-locking-type retractor (hereinafter abbreviated as ELR), that does not restrain the movement of an occupant during normal driving and, only when the vehicle undergoes an impact, locks a reel shaft of the seat belt to prevent the seat belt from being drawn out any more. When a deceleration equal to or greater than a predetermined value is detected, the ELR locks the reel shaft in a very short time and thereby prevents the seat belt from being drawn out any more.
However, in the case where merely the reel shaft is locked as in the above-mentioned ELR, if the seat belt restraining the occupant is wound loosely on the reel shaft, even if the reel shaft is locked, the seat belt is drawn out to some extent. Furthermore, even if the seat belt is not wound loosely, the seat belt is drawn out from the time that a deceleration equal to or greater than a predetermined value is detected until the reel shaft is locked.
For this reason, as a device that cancels out to some extent the drawing-out of a seat belt due to looseness of the seat belt or a time delay until the reel shaft is locked, there is known a pretensioner that pulls in a seat belt buckle when a predetermined deceleration is detected (for example, see Patent Document 1).
The pretensioner described in Patent Document 1 has a wire that is connected at one end to a seat belt buckle and fixed at the other end to a piston in a cylinder, and a housing that allows the wire to run therethrough and holds the cylinder. When a deceleration of the vehicle is detected, high-pressure gas is ejected to a gas chamber formed in the housing, thereby moving the piston and pulling in the wire. A guard pipe that protects the wire is disposed in the gas chamber in the housing.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-146184 (FIG. 2 and FIG. 4)